The present invention relates to a variable-valve-actuation (VVA) apparatus for internal combustion engines, and more particularly, to a VVA apparatus comprising a lift-amount varying mechanism for varying the lift amount of engine valves such as intake valve and exhaust valve and a lift-phase varying mechanism for varying the lift phase in the advance-angle or lag-angle direction.
As is well known, various VVA apparatus have been provided to use in combination a valve-lift adjusting mechanism (lift-amount varying mechanism) for varying the valve lift amount of, e.g. an intake valve and a valve-timing adjusting mechanism (lift-phase varying mechanism) for varying the lift phase or peak lift timing of the intake valve to enhance the degree of freedom of the valve-lift characteristics and thus largely improve the engine operating performance.
Specifically, a typical VVA apparatus comprises a valve-lift adjusting mechanism for selectively switching a low-velocity cam and a high-velocity cam mounted to a camshaft in accordance with the engine operating conditions for variable control of the cam lift for an intake valve or an exhaust valve, and a valve-timing adjusting mechanism for changing the relative rotation phase between the camshaft and crankshaft in accordance with the engine operating conditions for variable control of the lift phase of the valve.
In the VVA apparatus, when the valve-timing adjusting mechanism fails, the valve-lift adjusting mechanism forcibly switches the cam to the low-velocity side, whereas when the valve-lift adjusting mechanism fails, the valve-timing adjusting mechanism controls the opening/closing timing of the engine valve to have the valve-lift operation center away from the top dead center (TDC) of a piston. Such control allows prevention of interference between the piston and the intake valve or exhaust valve or between the intake valve and the adjacent exhaust valve.
With the above VVA apparatus, as described above, in the event of failure of the valve-lift adjusting mechanism, the valve-timing adjusting mechanism carries out control to have the valve-lift operation center away from TDC for prevention of interference between the intake valve and the adjacent exhaust valve, which, however, is carried out uniformly even during lift control of the low-velocity cam. This involves impossible approach of the valve-lift operation center to TDC during control of the low-velocity cam, failing to obtain fully advanced closing timing of the intake valve when the VVA apparatus is applied to the intake side. As a result, an effect of reduction in pumping loss is attenuated to make achievement of enhanced fuel consumption difficult.
Moreover, full enlargement of overlap of the intake valve and exhaust valve cannot be expected to make difficult achievement of enhanced fuel consumption due to increased residual gas in cylinders, etc.
Interference between the piston and the engine valve in the event of failure of the valve-timing or valve-lift adjusting mechanism can be prevented by increasing a valve recess in a piston crown face. However, this solution may cause remaining of unburned gas in the valve recess to lower the emission performance for exhaust gas such as HC.
It is, therefore, an object of the present invention to provide a VVA apparatus for internal combustion engines, which allows achievement of enhanced fuel consumption with excellent exhaust emission performance.
The present invention provides generally a VVA apparatus for an internal combustion engine, which comprises a first varying mechanism which controls a lift amount of an engine valve in accordance with engine operating conditions; a second varying mechanism which controls a lift phase of the engine valve in accordance with the engine operating conditions; a first sensor which detects an actual position of the first varying mechanism, the actual position corresponding to an actual lift amount; a second sensor which detects an actual position of the lift-phase varying mechanism, the actual position corresponding to an actual lift phase; and an ECU which controls the lift amount and the lift phase to first and second basic target values through the first and second varying mechanisms, respectively. When the actual lift amount exceeds the first basic target value by a predetermined value or more, the ECU corrects the lift phase through the second varying mechanism to separate from a TDC of a piston with respect to the second basic target value.